Luminescent phosphor screens are used in cathode ray tubes, for example, television display tubes, electron display devices, imaging devices, for example, image intensifier tubes, etc. Typically, a thin layer of phosphor material containing a luminescence activator is supported on a substrate. The phosphor layer is activated by impingement of an electron beam, and the resulting luminescence is transmitted through the glass substrate to the front of the display.
Phosphor screens, such as those used in image tubes, are typically made with phosphor powders, such as ZnS:AgZnCdS:Ag. The powder is applied to a substrate glass plate by any one of several known methods, such as setting, brushing, spraying, etc. However, the combination of a powdered phosphor layer on a glass plate has a main drawback of low resolution and low light output due to the scattering of emitted light in lateral directions among the phosphor particles. Other disadvantages include low adherence to the substrate, requiring the use of binder which complicates the production process, and that the powder is difficult to apply uniformly, leading to low process yields of acceptable units.
One proposal has been to apply powdered phosphors into intagliated (etched) recesses or wells formed in the fiber cores of an optical fiber bundle in order to increase the resolution of the screen. Such an intagliated powdered-phosphor screen is described, for example, in "Intagliated Phosphor Screen Image Tube Project", by Richard J. Hertal, ITT Aerospace/Optical Division, prepared for NASA under Contract NAS5-26417, May 1982. As illustrated in FIG. 1, the intagliated phosphor screen 10 was formed by etching the ends of the cores 13 of the fibers 11 of an optical fiber bundle 10 to about a one-diameter (10 micron) depth, then packing phosphor powder 20 into individual etched wells 13a. The isolation of the phosphor into individual etched wells 13a prevented the lateral spread of light that occurs in a single continuous phosphor layer. The walls of the wells 13a along the cladding sheaths 12 of the fibers could be metallized to enhance the isolation effect. However, while the resolution of the phosphor screen was improved by the intagliated wells, the light output was quite low, even with metallization of the cladding walls, and the intagliated wells required a cumbersome etching process.
Another approach to improving the resolution of a phosphor screen is by deposition of the phosphor as a thin film or monocrystalline layer on a substrate. Such thin film phosphor screens have relatively high resolution, but have the disadvantage of large internal reflection losses. Due to a difference in refraction indices at the phosphor/substrate interface and the lateral waveguide effect of the thin film layer, the optical output for thin film phosphor screens have been only about 5% to 10% of the light emitted.
Some researchers have proposed etching grooves, trapezoidal mesas and other reticulated structures in the thin film phosphor layer and providing a reflection coating thereon, in order to break up the waveguide effect and enhance light output. Such reticulated structures are disclosed, for example, in U.S. Pat. No. 4,298,820 to Bongers et al., and in the article entitled "Reticulated Single-Crystal Luminescent Screen", by D. T. C. Huo and T. W. Hou, Journal of Electrochemical Society, Vol. 133, No. 7, pp. 1492-97, July 1986. The reticulated thin film structures have improved the light output of the resulting phosphor screen. However, they generally require high lithography resolution, and the crystalline phosphors tend to etch along crystalline planes which are different from the optimum light containing slope angle. Thus, the application of reticulated thin film phosphor layers has also been limited.